Vehicles include transmissions to convert and transmit driving force produced by engines according to a driving state of the vehicle. The transmissions include manual transmissions in which a plurality of gear trains are shifted by manual operation, and automatic transmissions in which the gear trains are shifted automatically according to a driving state.
In the manual transmission that is equipped with gear trains and a shift mechanism, the shift mechanism is operated manually to change to one of the gear trains. The automatic transmission, on the other hand, is equipped with a torque converter and an auxiliary transmission in which gears are automatically shifted by an actuator according to the driving state.
In the automatic transmission with the torque converter, the torque converter is used while the vehicle is started and traveling, and the gear is shifted by a clutch. The torque converter on the automatic transmission, however, has low efficiency of drive force transmission and a large capacity oil pump is provided to supply oil pressure to a clutch transmission, and to the torque converter. As a result, loss of the driving force increases fuel consumption.
To eliminate the aforesaid problem, an automatic transmission is provided which is based on the manual transmission including gear trains and shift mechanism and which automatically shifts the gear trains by the actuator.
One example of the automatic transmission based on the manual transmission is shown in FIG. 16. In FIG. 16, reference numeral 202 designates an engine, and 204 the automatic transmission. The automatic transmission 204 includes: an input shaft 210 connected to a crankshaft 206 of the engine 202 through a clutch 208; an output shaft 212; gear trains 214, 216, 218, 220, 222 and a reverse gear train 224; synchronizer-type shift mechanisms 226, 228, 230; a reverse shift mechanism 232; a final gear 234; a differential 236; and a drive shaft 238. The drive shaft 238 is connected to drive wheels (not shown) of the vehicle.
In the automatic transmission 204, an automatic gearshift device (not shown) automatically changes the gear trains 214, 216, 218, 220, 222. During gearshift, an actuator (not shown) operates to start the clutch 208, the 1st/2nd shift mechanism 226, the 3rd/4th shift mechanism 228, the 5th shift mechanism 230, and the reverse shift mechanism 232 so that the gear trains 214, 216, 218, 220, 222 and 224 are shifted according to a driving state of the vehicle.
In the automatic transmission 204 based on the manual transmission, the clutch 208 is to be disengaged during gearshift from first to second speed with engagement of gear being shifted from the first speed gear train 214 to the second gear train 216, and during gearshift from second to third speed with engagement of gear being shifted from the second speed gear train 216 to the third speed gear train 218. This disengagement of the clutch 208 results in disconnection of acceleration torque to the output shaft.
Another example of an automatic transmission based on the manual transmission to deal with this problem is disclosed in Japanese Patent No. 2703169. The automatic transmission is provided with a sub or secondary clutch which can variably control transmitted torque and which is disposed on a torque transmission path defined by a gear train having a minimum transmission gear ratio between the input and output shafts. During gearshift, the sub clutch transmits driving force from the input shaft to the output shaft. Thereby, disconnection of the acceleration torque to the output shaft can be prevented.
An automatic transmission equipped with such a sub clutch is also disclosed in JP Laid-Open No. 2001-227599. This automatic transmission is provided with a sub clutch which can variably control transmitted torque and which is disposed on a torque transmission path defined by gear trains corresponding to third to fourth speed between input and output shafts. The sub clutch transmits driving force from the input shaft to the output shaft during gearshift so as to prevent the disconnection of the acceleration torque.
Referring to dashed lines in FIGS. 3, 5, 7, 9, in the automatic transmission of the first example (i.e. FIG. 16), the clutch 208 is disengaged during each gearshift of 1st to 2nd, 2nd to 3rd, 3rd to 4th, and 4th to 5th speeds. Then after completion of synchronization of each shift mechanism 226, 228, 230; each shift sleeve 240, 242, 244 is engaged with the respective selected next gear, and then the clutch 208 is engaged. As a result, the acceleration torque during acceleration is disengaged or disconnected according to the first example of the automatic transmission. In particular, the acceleration torque is reduced or drops considerably during the gearshifts from 1st to 2nd speeds and from 2nd to 3rd speeds. The larger the drop in acceleration torque, the larger the increase of time for proper synchronization, resulting in noticeable disengagement or disconnection of the acceleration torque, and detectable uncomfortable feeling during the gearshift.
In contrast, referring to dot-dash and dashed lines in FIGS. 3, 5, 7, 9, in the automatic transmission of the second example, during each gearshift of 1st to 2nd, 2nd to 3rd, 3rd to 4th, and 4th to 5th speeds, the clutch 208 is not disengaged and maintains transmitting of the driving force from the input shaft to the output shaft. Then after completion of synchronization, each sleeve of the shift mechanism is engaged with the respective selected next gear. As a result, the acceleration torque during acceleration is not disengaged according to the second example of the automatic transmission. In this second example of the automatic transmission, however, the sub clutch to maintain the acceleration torque during gearshift is disposed in the gear train having minimum gear transmission ratio, which results in a lower acceleration torque maintained by the sub clutch.
In the automatic transmission of the second example, the acceleration torque drops considerably, since the acceleration torque maintained is small, in particular during gearshift from 1st to 2nd speed, which has the large gear transmission ratio and the largest acceleration torque, and from 2nd to 3rd speed. This does not distinguish from the first example with regards to driver's uncomfortable feeling owing to reduction of the acceleration torque during the gearshift.
In addition, the automatic transmission of the second example does not change the constitution of the automatic transmission 204 of the first example shown in FIG. 16 having the gear trains 214, 216, 218, 220, 224 and the shift mechanisms 226, 228, 230, 232, except for the additional sub clutch. This increases the number of components, i.e., the additional sub clutch, and increases cost.
The automatic transmission disclosed in above-mentioned JP Laid-Open No. 2001-227599 includes a gear train corresponding to the gear transmission ratio from 3rd to 4th speed dedicated to the sub clutch. However, the automatic transmission does not change the constitution of the automatic transmission 204 of the first example shown in FIG. 16, except for the additional sub clutch and an intermediate shaft for the sub clutch, and input and output gears to transmit the driving force by the sub clutch. This increases the number of components and increases cost.